1. Field of the Invention
The present invention generally relates to the detection of RF signals and, more particularly, the detection of digital television signals.
2. Background of the Invention
Frequency reuse plans oftentimes are implemented to increase the efficiency of RF spectrum usage. Traditional frequency reuse plans generally are limited to licensed users who reuse frequencies within RF spectrums they have licensed. New plans, however, propose expanding frequency reuse to allow unlicensed users to use RF spectrums that potentially may be licensed to other users, referred to as incumbent users, so long as the unlicensed users do not interfere with communications of the incumbent users. For example, unlicensed users in a particular geographic region may utilize RF spectrums that are reserved for digital television (DTV) broadcasts if there are no broadcasters presently using these RF spectrums.
Unless an unlicensed user has prior knowledge of which RF spectrums currently are not being used, the unlicensed user must determine whether a particular RF spectrum is available before transmitting on that RF spectrum. For instance, if the unlicensed user wants to transmit in a RF spectrum normally reserved for DTV broadcasts, the unlicensed user must determine that the desired RF spectrum is clear of such broadcasts.
One way to determine whether a RF spectrum is clear of DTV broadcasts is to determine whether any DTV pilot signals are present. In DTV broadcasting, a pilot signal is generally used to provide, among other things, a synchronization reference for intended receivers. A pilot signal usually comprises an RF signal transmitted at a single frequency and with a particular amplitude. For example, an Advanced Television Standards Committee (ATSC) compliant pilot signal for a DTV broadcast typically is transmitted at approximately 309 kHz above the lowest frequency of the broadcast's allocated RF spectrum, and a power level that is approximately 11 dB lower than that of the DTV total transmitted signal power.
As with most types of RF signals, the receive signal strength of a pilot signal and its corresponding DTV broadcast can vary due to a number of influences, including RF noise, geography, obstacles along the RF propagation path, weather conditions, and so on. Thus, the receive signal strength of a pilot signal may not always measure 11 dB less than its corresponding DTV broadcast. Moreover, it is sometimes difficult to discriminate the pilot signal from RF noise and other RF signals that may be present. Thus, identification of DTV pilot signals can be imprecise. Indeed, in some instances existing DTV pilot signals may not be identified, and signals which are not DTV pilot signals may be mistakenly identified as such.